Method for producing light metal castings and casting mold for carrying out the method

ABSTRACT

A method for producing light metal castings, in particular cylinder heads, cylinder blocks and/or crankcases for internal combustion engines, includes a sand mold which forms a mold cavity for the casting. The sand mold includes outer mold parts, at least one core and at least one feeder for forming a riser, wherein the mold is provided with an in-gate for receiving the metal melt and the metal melt is filled into the mold cavity under the effect of gravity. A cover core (5) is fitted onto the mold that is at least in partial regions designed to be impermeable to gas and which contains at least one feeder, and wherein immediately after the filling operation, the feeder filled with metal melt is admitted with a pressure via a pressurized gas.

BACKGROUND OF THE INVENTION

The invention relates to a method for producing light metal castings, inparticular cylinder heads, cylinder blocks and/or crankcases forinternal combustion engines, with a sand mold for forming a moldingcavity for the casting, which comprises outer mold parts, at least onecore and at least one feeder for forming a riser, wherein the mold isprovided with an in-gate for the metal melt and wherein the metal meltis filled into the molding cavity of the mold under the effect ofgravity.

It is a known practice to use sand molds and permanent molds, so-calledchill molds, for the production of light metal castings, as well as touse molds that are composed of permanent mold parts, so-calledhalf-chills, and sand-casting mold parts. On the one hand, the mainadvantage of the series production of castings with the sand mold liesin this case in the high production speed for producing the sand moldsand, on the other hand, in the high production speed for the castingwith respect to the casting rate. In contrast to the casting withchill-molds, it is not necessary after each casting operation to waitfor the mold to cool down before reusing it. Since it is also possibleto recycle the sand used for producing the outer mold parts and thecore, light metal castings such as cylinder heads for internalcombustion engines, are nowadays cast at high productivity in sandmolds. In this case, the metal melt is always poured into the sand moldunder the effect of gravity, for the most part with the so-calledgravity casting. To improve the texture, feeders are used at the mold,which contain a feeder material that matches the material of the castingfor high-quality castings and functions as a so-called riser. For this,the somewhat coarser texture as well as the possibly occurringmicro-porosity when casting in sand-based molds as compared to thecasting in chill molds is accepted.

Furthermore a known method of low-pressure casting of light metal meltsin permanent chill molds, wherein a fine, dense texture can be achievedby applying pressure with a gas to the melt inside the chill mold isdisclosed in DE-A-21 33 421; and DE-A-28 18 442. The application ofpressure is no problem because of the stability of the chill mold.However, since the chill molds have a very long cooling-down period, anextremely high number of very expensive chill molds are needed for highproduction speeds. Owing to the frequently occurring changes in thecastings, a high number of chill molds must therefore be changed, or newones must be produced.

From the DE-B-11 35 619 publication it is known to use a standard sandmold with a fixed upper frame and a fixed lower frame, wherein for theproduction of metal castings with a dense structure, the sand moldconsisting of upper and lower frames is covered with a sealed dome orthe upper and lower frame of the mold are connected such that they aresealed and a dome that is also sealed is placed over the free surface ofthe upper frame after the molding cavity of the mold has been filledwith metal melt. The inside space of the dome is admitted with apressurized gas, which then acts upon and compresses the melt via theporosity of the mold sand, in particular via the free melt surfaces inthe region of the feeders and risers. This method is time-consumingdespite the use of sand molds since either a complete dome must beplaced in each case over the sand mold or care must be taken during thefitting together of the sand mold so that lower frame and upper frame aswell as the pressure dome that is fitted on top are sealed completelywhen connected.

From U.S. Pat. No. 2,960,736 it is known that in order to solve thesealing problems, a pressure dome is fitted over the feeder opening,which is provided with a sealing collar that dips into the melt insidethe feeder. The disadvantage of this system is, among other things, thata relatively large melt surface must exist in the end section of thefeeder for sealing purposes, independent of the size of the castings tobe cast, so that following the separation of the feeders from thefinished casting a correspondingly large amount of material must bemelted on again.

SUMMARY OF THE INVENTION

It is the object of the invention to improve the process of castinglight metal castings under the effect of gravity and in sand molds withrespect to the quality of the produced casting, in particular concerningan especially dense and fine-grained texture while avoidingmicro-porosity, as well as to reduce the volume of the feeder materialaccruing in the form of a riser. Light metal within the meaning of theinvention comprises in particular aluminum and aluminum alloys.

The solution according to the invention, with respect to the processingmethod, is that a cover core is fitted on the mold, which isgas-impermeable at least in partial regions, and which comprises atleast one feeder, and that immediately after the filling operation, thefeeder filled with metal melt is admitted with a pressure via apressurized gas.

With the inventive processing method, it is possible even with sandmolds to improve, condense and refine the texture of the solidifyingmetal melt through additionally admitting it with pressurized gas viathe riser, as the arrangement of an air-impermeable cover core providesan immediate option for supplying the pressurized gas. At the same time,the volume or mass of the riser can be reduced by one half as comparedto traditional casting methods with dead molds, so that a high savingsin the circulating material can be achieved. In addition to the literalmeaning for gas-impermeability, the term "gas-impermeable" within themeaning of the invention at hand also refers to gas-permeability with ahigh flow resistance, which for a pressure admitting ensures the desiredpressure build-up above the melt with acceptably low losses throughleaks.

Advantageous modifications of the inventive method will become apparentto those skilled in the art upon reading the following description. Onepreferred embodiment of the inventive method provides that the metalmelt is admitted with pressure inside the feeder during the firstcritical solidifying phase for the metal melt, meaning while the metalmelt is still hot, so that a corresponding influencing of the stillliquid melt in the total mold space via the exerted pressure ispossible. This first, critical solidifying phase extends approximatelyto a cooling temperature for the light metal melt of 500° C. Below thistemperature, the admitting with pressure has only a limited effect.

In order to permit in a simple way the admitting of the metal melt inthe feeder with pressure at the point in time when the filling operationis completed, but the metal melt has not yet solidified, it is suggestedto section off a hollow space above the feeder and the cover core thatcontains the feeder, to seal off this space and provide it subsequentlywith a connection for admitting it with pressurized air. After theadmitting of the feeder with pressure is completed, this sectioned-offhollow space can be used in a further processing step to apply anegative pressure to this space above the feeder and, at the same time,to vacuum off the gases that escape during the hot-casting from themold.

In accordance with the invention, a (lost) mold is further suggested forcarrying out the inventive method, which is characterized by outer moldparts, at least one core as well as at least one feeder for forming ariser, wherein a cover core with at least one feeder is provided, ontowhich a pressure dome is fitted that seals off the feeder region andwherein the cover core is designed to be gas-impermeable, at least inthe region where the pressure dome is fitted on.

In a further modification of the inventive mold and for an accurateadmitting with pressure, even with a sand-based cover core, it issuggested that the sand-based cover core that contains the feeder ismade impermeable to gas by a layer of core black wash, applied to itssurface at least in the region where the pressure dome is fitted on. Thecover core itself can be composed of several individual cores to form acover core, wherein each individual core must be designed to begas-impermeable and wherein the individual cores must be connectedtightly, at least in the feeder region. After the mold has been filledwith metal, this cover core or the cover cores make it possible to applya pressure by means of pressurized gas onto the feeders installed in thecover core, in order to achieve in this way an improved, more refinedand more dense texture of the produced casting with simultaneously lowerfeed volume--riser volume--during the solidification.

Another advantageous embodiment provides that as a result of acorresponding selection of the mold material and the binder, the covercore is designed such that it can withstand a pressure of up to 1 bar.In order to carry out the inventive method, only low pressures below andup to 1 bar are needed to achieve the desired density and fine-grainedtexture.

In a further modification of the invention, the pressure dome providedon the cover core for producing a hollow space for the admitting withpressure can also be used to vacuum off the smoke that develops as aresult of the thermal effect of the metal melt on the mold sand and thecore sand. It must therefore be designed such that immediately followingsolidification of the metal melt and after completion of the admittingof the riser with pressure, a low pressure can be applied to the hollowspace covered by the pressure dome. The pressure dome is provided withanother connection for this, to which a vacuuming device can beattached.

One advantageous embodiment of the cover core seal involves theapplication of a core black wash as dispersion by means of immersion orspraying on. The surface treatment with a core black wash also resultsin an improved firmness of the cover core. Core black washes are knownin principle as finishes for sand-based cores to achieve smooth andnon-porous surfaces when producing castings from a gray cast iron.

Core black washes on the basis of a dispersion of fire-resistant fillermaterials in an organic solvent are provided as inventive finishes for acover core so as to make it impermeable to gas. These core black washescan additionally contain graphite. The fire-resistant filler materialsto be considered can include, for example, fire-resistant silicates suchas zirconium silicate, magnesium and/or aluminum silicate. In addition,the core black wash can also contain iron oxide as fire-resistant fillermaterial. The core black washes can furthermore contain small amounts ofsynthetic resins, that is up to 1% in weight.

The previously explained dispersions are possible black washes, butother material compositions can be used as well, which result in agas-impermeable surface for the sand-based cores.

In order to further improve the texture for the inventive, sand-basedmold for light metal castings, it is suggested to assign supportingshells to the outer mold parts of sand, which at least partiallysurround the mold and can be moved toward or away from the mold. Thesupporting shells that are preferably made of a permanent material, forexample metal, function as outside pressure supports on the casting moldduring the casting operation and to admit pressure during the criticalfirst solidification phase of the metal melt in the casting mold.

The inventive casting mold permits a new and economical method forproducing light metal castings through pouring by gravity, in particularalso of such complicated parts as cylinder heads and cylinder blocks forinternal combustion engines with a high casting quality by usingsand-based molds for the outside mold as well as for the cores. It ispossible to produce a dense, fine and even texture for the completecasting by way of the pressure, which is additionally exertedimmediately after the completion of the casting operation onto thecasting via the metal melt in the feeder, so that on the one hand theadvantages of the sand-based lost mold can be utilized and, on the otherhand, a high quality is achieved, such as normally can be achieved onlyfor castings with a chill mold.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in more detail with the aid of diagrammaticdrawings of exemplary embodiments, in which:

FIG. 1 is a diagrammatic illustration of a casting mold;

FIG. 2 Illustrates the fitting of the casting mold according to FIG. 1with a pressure dome;

FIG. 3 A diagrammatic, exploded view of a casting mold for a crankcase,as seen from the side;

FIG. 4 is a diagrammatic illustration of the casting mold according toFIG. 3 in the assembled state;

FIG. 5 is a casting mold according to FIG. 1 with additional bell forsuctioning.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 is a cross-sectional view of a casting mold 1 for a simplerotation-symmetrical casting with a molding cavity 2. The mold 1 isdesigned as a lost mold with mold outside parts 3.1, 3.2 as well as aninserted core 4 and has a cover core 5, comprising feeders 6 for formingthe riser. The mold outside parts 3.1 and 3.2 are joined at theseparation joints 7. The core 4 can also be composed of several parts toform a core packet. The number and subdivisions of the mold outsideparts and the produced cores 7 or core packets depend on the respectiveform in which the casting is to be poured. The casting mold 1furthermore has an in-gate 8 for the metal melt. The cover core 5 aswell can be fitted together of several core parts and can have one orseveral feeders 6.

FIG. 2 shows the mold 1 after the metal melt has been poured in to fillthe molding cavity and to produce a casting 9, wherein the riser 10 arepresent in the feeders for the cover core 5.

In order to achieve a condensing and evening out of the texture and toavoid micro-porosity after the mold cavity has been filled with metalmelt, a gas pressure P is exerted via the feeders 6 onto the metal meltthat forms the riser 10. This pressure P can range up to 1 bar.

A pressure dome 11 is fitted onto the cover core 5 such that it seals tobe able to exert this pressure via the feeders 6 onto the metal melt inthe mold cavity until solidification. The pressure dome 11 has aconnection 12 for the compressed air supply.

The cover core 5 produced in the known way on the basis of sand isnormally microporous and thus air-permeable. In order to be able toexert pressure onto the still liquid metal melt by means of compressedair via the feeders 6, the cover core 5 surface is coated with a coreblack wash, at least in the region covered by the pressure dome 11, thusmaking it impermeable to gas in the coated region. The cover core 5surface can be sealed, for example, by submerging the cover core 5 intoa core black wash. With the aid of the core black wash, e.g. on thebasis of a dispersion of fire-resistant filler materials in an organicsolvent such as zirconium silicate and carbon, small amounts ofsynthetic resin and isopropanol diluted with acetone and containing somewater, the surface of the cover core 5 is made so dense and stable thatthe cover core 5 is air-impermeable and can withstand the added pressureP exerted via the pressure dome, as shown in the diagram in FIG. 2.

Essentially, it is only necessary to seal and render air-impermeable thecover core 5 by coating all those regions which come in contact with thecompressed air. However, the submerging of the complete cover core intoa respective core black wash is an economical method, which not onlyresults in a corresponding sealing of the cover core surface, but also ahigher stability.

If the cover core 5 is composed of several parts, then each part must besealed with a core black wash prior to assembling.

The core black wash applied to the cover core must dry and solidifybefore the cover core is used.

The pressure P is applied to the casting by means of the pressure dome11 fitted onto the mold until the solidification process for the lightmetal melt is finished, i.e. until a temperature below 500° C. isreached. This time depends on the size of the workpiece that isproduced, e.g. approximately 1.5 minutes for a cylinder block forinternal combustion engines with a casting weight of approximately 20 to22 kg.

In FIG. 3, the use of the invention for producing a crankcase forinternal combustion engines is shown is an exploded view of thenecessary casting mold parts. The outer casting mold is here formed bythe upper mold block 3.4, the side mold parts 3.5 as well as the lowermold parts 3.6, which can also be composed of individual core parts,depending on the configuration. The core block 4.1, which is also calledcrankcase core block, is again composed of individual core parts. Thecover core 5 forms the upper part of the mold. All casting mold parts aswell as the core block 4.1 and the cover core 5 are produced assand-based lost molds. The cover core 5 here can have an additional part13, namely a cylinder liner, that later on remains in the casting.

The cover core 5 contains at least one feeder 6 for the riser and isfurthermore covered on the top with the pressure dome 11. The pressuredome 11 is fitted such that it seals onto the cover core 5 and thehollow space 14 thus created over the cover core 5 can then be admittedwith a pressure via the compressed air that is fed in via the connection12, so that a pressure can additionally be exerted in the-feeding regiononto the metal melt inside the mold until solidification.

The cover core 5 can also be located on the opposite side of thecrankcase, so that the mold is turned by 180°.

Supporting shells 16, which act upon the outer mold parts 3.4, 3.5, 3.6,can be provided to stabilize the casting mold during the castingoperation to increase the form precision, and to exert an additionalpressure from the outside onto the casting mold. These supporting shells16 can, for example, be folded down at the casting location prior toremoving the casting and after solidification.

FIG. 4 shows a cross-sectional diagram of the sand-based lost molddescribed in FIG. 3 for a crankcase, in the assembled condition. Thearrow 17 designates the direction in which the extractors work after thesupporting shells 16 have been moved to the side. Here too, the covercore 5 is coated on its surface with a core black wash and treated tomake it air-impermeable, thus permitting the admitting with a pressureof up to 1 bar via the pressure dome 11.

After admitting the cover core 5 with pressure, meaning after this iscompleted, it is possible via a second connection 18 to suction off fluegases that have collected in the hollow space 14 formed by the pressuredome 11, which gases escape and collect from the mold parts inparticular through the effects of the hot melt.

FIG. 5 shows a modification of the invention, wherein another suctionbell 19 is fitted at a distance over the pressure dome 11, so that thissecond suction bell covers the upper part of even the outer mold partregion, so that all flue gases developing as a result of the castingheat and which escape from the mold upward can be suctioned off fromunder the suction bell 19 via the connection piece 20. It is possible toconnect the pressure dome 11 and the suction bell 19 via supports 21.The pressure dome 11 as well as the suction bell 19 are fitted onto thecover core 5 or the outer mold parts such that they form a seal. Theconnections 12 and 20 are connected to respective compressed gas feedand suction devices.

I claim:
 1. A method for producing light metal castings, comprising the steps of:obtaining a sand mold to form a mold cavity for the casting, said sand mold including outer mold parts, and at least one sand-based core; fitting a sand-based cover core onto the sand mold to cover the mold cavity, said sand-based cover core being gas-impermeable at least in partial regions and comprises at least one feeder for receiving a pressurized gas and in which a riser is formed; filling the mold cavity with metal melt through an in-gate located in one of the sand mold or the sand-based cover core, the metal melt flowing into the mold cavity under the effect of gravity; and following the metal melt filling step, sealingly attaching a device for supplying the pressurized gas to the gas-impermeable partial regions of the sand-based cover core and applying pressure to the at least one feeder filled with metal melt via the pressurized gas.
 2. The method according to claim 1, wherein the light metal castings produced are cylinder heads, cylinder blocks and/or crankcases for internal combustion engines.
 3. The method according to claim 1 wherein the at least one feeder, which is filled with metal melt, is admitted with a pressurized gas up to about 1 bar.
 4. The method according to claim 1, wherein the cover core is made gas-impermeable by applying a core black wash to at least partial regions of the cover core.
 5. The method according to claim 1, wherein the metal melt in the at least one feeder is admitted with pressure during the first critical solidification phase of the metal melt.
 6. The method according to claim 1, wherein the applying pressure step continues until the metal, which has been poured into the mold, has reached a temperature below 500° C.
 7. The method according claim 1, wherein the device which is sealingly attached to the cover core forms a hollow space above the cover core, the hollow space being admitted with the pressurized gas in order to apply pressure to the at least one feeder.
 8. The method according to claim 7, wherein after the applying pressure step has ended, a negative pressure is applied to the hollow space.
 9. A method for producing light-metal castings, comprising the steps of:obtaining a sand mold to form a mold cavity for the casting, the sand mold comprising outer mold parts, at least one sand-based core and at least one feeder for forming a riser, a sand-based cover core that is gas-impermeable at least in some sections comprises the at least one feeder and is sealingly fitted onto the sand mold; filling the sand mold with metal melt through an in-gate, the metal melt flowing into the mold cavity under the effect of gravity; and immediately following the filling of the metal melt into the mold cavity, applying pressure to the at least one feeder filled with metal melt by admitting it with a compressed gas wherein a feeding device for the compressed gas is attached to the gas-impermeable partial sections of the sand-based cover core so as to form a seal.
 10. A casting mold for producing light metal castings, comprising:a sand mold to form a mold cavity for the casting, said sand mold including outer mold parts and at least one sand-based core; a sand-based cover core with at least one feeder for receiving a pressurized gas and in which a riser is formed, said sand-based cover core being fitted onto the sand mold to cover the mold cavity; and a pressure dome that is sealingly attached to the sand-based cover core to seal off a pressurized gas feeding region, wherein the sand-based cover core is gas-impermeable, at least in the region covered by the pressure dome.
 11. A casting mold according to claim 10, wherein the cover core is sand-based and is made gas-impermeable by applying- a core black wash to its surface, at least in the region covered by the pressure dome.
 12. A casting mold according to claim 11, wherein the sand-based cover core can withstand a pressure of up to 1 bar during the admitting of the feeding region with pressure.
 13. A casting mold according to claim 11, wherein the core black wash is applied in the form of a dispersion to the cover core by means of dipping or spraying.
 14. A casting mold according to claim 10, further comprising support shells which are assigned to the outer mold parts, said support shells encircling the outer mold parts at least in part and can be moved toward or away from the sand mold.
 15. A casting mold according to claim 10, further comprising a connection for suctioning off gases, said connection being attached to said pressure dome.
 16. A casting mold according to claim 10, further comprising a bell, which can be fitted over the pressure dome onto the sand mold such that it seals and spans at least the cover core, said bell being provided with a connection for admitting a negative pressure inside the bell. 